Known abrasion resistant cable, such as steel wire armoured cable, has good abrasion resistance and is strong enough to be suspended from a cable laying vessel during laying in water deeper than 5 km.
Such abrasion resistant cable is conventionally used when the section of seabed upon which the cable is being laid includes a zone of abrasion risk.
Once the zone of abrasion risk is passed, abrasion resistant cable is no longer needed; continuing the abrasion resistant cable beyond this zone would be prohibitively expensive. The abrasion resistant cable therefore needs to be connected, by joint or transition, to a cheaper, but lighter and consequently weaker, conventional un-armoured cable.
However, deeper than about 2 km, generally the weight of steel armoured cable is too great to safely permit it to be supported during suspension by the weaker unarmoured cable joined thereto. This means that it is not possible to directly lay from armoured to un-armoured, or recover from un-armoured to armoured cable.
It is still possible to install the armoured cable in water deeper than 2 km, using an appropriate installation sequence where the strong armoured cable always supports the weaker un-armoured cable, and never vice-versa. However, such a sequence requires good weather, additional joints, cable and considerable time. It is accordingly both risky and expensive.